Re: [PATCH] IB/ipoib: move back the IB LL address into the hard header

From: Paolo Abeni <redacted>
Date: Tue, 11 Oct 2016 19:15:44 +0200

After the commit 9207f9d45b0a ("net: preserve IP control block
during GSO segmentation"), the GSO CB and the IPoIB CB conflict.
That destroy the IPoIB address information cached there,
causing a severe performance regression, as better described here:

http://marc.info/?l=linux-kernel&m=146787279825501&w=2

This change moves the data cached by the IPoIB driver from the
skb control lock into the IPoIB hard header, as done before
the commit 936d7de3d736 ("IPoIB: Stop lying about hard_header_len
and use skb->cb to stash LL addresses").
In order to avoid GRO issue, on packet reception, the IPoIB driver
stash into the skb a dummy pseudo header, so that the received
packets have actually a hard header matching the declared length.
Also the connected mode maximum mtu is reduced by 16 bytes to
cope with the increased hard header len.

After this commit, IPoIB performances are back to pre-regression
value.

Fixes: 9207f9d45b0a ("net: preserve IP control block during GSO segmentation")
Signed-off-by: Paolo Abeni <redacted>

Not providing an accurate hard_header_len causes many problems.

In fact we recently fixed the mlxsw driver to stop doing this.
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